I have an ex-internet friend ....And she has my address and I am quite sure is following me on various social media and other message boards. I get a bad feeling that she could show up on my doorstep one day.
Block her on every social media site you can!
Make sure you are on a friendly basis with some of your neighbors, or save the number of a friend who lives nearby so you have someone to ask for help if she does ever show up. (She will probably
never show up, but it will make you feel safer.)
In person, I can tell before the person even speaks. It is a vibe. Body language, posture, movements and when they speak the phrasing and tone they use set off alarms. It is really easy for me to decipher whether someone is harmless and odd, or a creep IRL.
This could be any kind of creep: sexual predator, thief, bully, scammer, someone out to be hurtful for entertainment.... etc.
People who are too pushy with attention, gifts and compliments, but with a threatening vibe, not because they are just "nice". Either they are using the gifts as a trojan horse of sorts, or they are trying to bribe you to give them attention, and if you reject them they turn threatening.
Online it is a little harder, which is why I am not as open as I could be, despite wanting to be friendly and inclusive with everyone.
A lot of creeps reach out and try to be friends, but the moment they find out you are not interested or available, they become hostile. The hostility can be masked in humor, like they are still trying to be "pseudo-friendly", but it is still there. Sometimes it is pure hostility, too.
Sometimes this feels dangerous, other times it seems like a protective shell they are putting up around themselves to keep from being hurt over rejection.... either way, they're jerks, and I stay away.
With stalkers: I had two stalkers when younger, both of whom had a feverish attachment to me right away. You can see it in their eyes - like a dog watching a squirrel. That is when you need to back away, and fast.